1. Field of the Invention
The present invention relates to a downshift control device for an automatic transmission of a motor vehicle, and more particularly to a technique for controlling pressures applied to and released from engaging elements by controlling back pressures of accumulators in the process of shifting down the transmission while an accelerator pedal is being depressed.
2. Description of the Prior Art
There is known a shift control device for an automatic transmission as disclosed in Japanese laid-open Patent Publication No. 2-80850.
In order to reduce shift shocks during power-on downshifting, the control device disclosed in the above-identified publication is provided with delaying means for delaying engagement of a first input clutch that is to be engaged as a result of downshifting, i.e., means for keeping a small increase in the pressure applied to the first input clutch in a later period of shifting, so as to prevent the first input clutch from being engaged with a transmission torque capacity before first one-way clutch is engaged such that the rotating speed of an input shaft of the transmission matches the gear position in which the transmission is to be placed after the downshifting.
Since the above-described known downshift control device employs the one-way clutch for optimizing the downshift timing in which the input and output shafts of the transmission are rotated in synchronization with each other, pull-in or drawing shocks can be avoided and desirable downshifting can be thus achieved by suitably controlling the pressure to be applied to the clutch during downshifting and reducing the increase in the rotating speed of the input shaft of the transmission. However, in an automatic transmission equipped with a power transmission mechanism in which the one-way clutch is eliminated and the downshift timing of synchronized rotation is controlled by increasing the pressure applied to the clutch, the rotating speed of the input shaft of the transmission cannot be controlled by controlling the pressure applied to the clutch.
In the automatic transmission equipped with the power transmission mechanism in which the one-way clutch is eliminated, therefore, when the clutch pressure to be applied is increased earlier than the synchronization timing (as indicated in 1 in FIG. 10), by keeping the engine speed N.sub.E (turbine speed N.sub.T) being increased at a high rate (steep slope of increase in the engine speed N.sub.E) throughout the downshift operation, the rotation of the input shaft is pulled up and the output torque T.sub.Q becomes temporarily negative, resulting in pull-in shocks. These shocks can be reduced where the clutch pressure to be applied is increased around the synchronization timing (as indicated in 2 in FIG. 10). Where the clutch pressure to be applied is increased later than the synchronization timing (as indicated in 3 in FIG. 10), however, the rotation of the engine that is racing is pulled down, and the output torque T.sub.Q is temporarily raised to a great extent, resulting in thrust shocks.
As described above, where the slope of increase in the engine speed N.sub.E (turbine speed N.sub.T), i.e., slope of increase in the gear ratio, largely changes even in a region around the synchronization timing, the clutch pressure to be applied must be increased in the timing that almost corresponds to a point of time or a moment. Where rotating conditions, oil temperature and other conditions slightly differ each time the transmission is shifted down, therefore, the clutch pressure to be applied cannot be always increased in the same timing as the synchronized rotation, and the pull-in shocks or thrust shocks cannot be thus avoided.